Quite common on forums and useful, e.g. you look at all forum topic titles and decide OK, you've seen enough of the new stuff. This function doesn't seem to exist, which is especially bad timing now with the introduction of unread markers and the resulting fact that approximately 99.99% of all forums threads will be unread to you. And who reads them all anyway? So you look at a thread list and only see the threads you looked into and have those as indication what you don't have to check out if no new activity is in there. But every time you come back, the same unvisited threads that you're not interested in will still crave for your attention even if there was no new activity in them. Admittedly this can be somewhat managed by using "newest" thread sorting, so this is not a strong request, but an incentive to ponder on such a feature. (I'm too tired and underpaid right now to do a complete feasibility study.)